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Trends in Atlantic surfclam (Spisulu solidissima) population demographic parameters were analyzed using age and length observations obtained from NOAA stock surveys conducted from the 1980s to 2010s in six regions distributed along the Middle Atlantic Bight (MAB) continental shelf. Atlantic surfclam asymptotic length and specific growth rate were estimated for each survey region and decade using the von Bertalanffy growth function. Specific mortality rates were estimated using a linearized negative exponential relationship and the maximum Atlantic surfclam age observed in each survey region for each decade. The estimated Atlantic surfclam mean length in the southern regions of the MAB decreased from 127 to 103 mm, about a 19% decrease, over the four decades. The mean length remained stable at about 130-135 mm in the central survey regions in contrast to the mean length of the Georges Bank Atlantic surfclam population which showed an increase from 101 to 135 mm over the four decades. The asymptotic length estimated for the southernmost survey region declined by 20% and remained relatively constant for the other survey regions over the four decades. Estimated specific growth rates remained unchanged for most of the survey regions, with the overall regional mean decreasing from about 0.25 [y.sup.-1] in the 1980s to 0.20 y~' by the 2010s. Specific mortality rates were not significantly different across the survey regions, although the maximum age estimated for the population in the southern survey region decreased by about 36%. This study provides a quantitative assessment of past and ongoing changes in MAB Atlantic surfclam population demographics that can be used to develop projections of responses to stresses imposed by climate change and commercial fishing. KEY WORDS: Atlantic surfclam Spisulu solidissima, population demographics, growth rale, mortality rate. Middle Atlantic Bight, INTRODUCTION The Middle Atlantic Bight (MAB) region, the portion of the Northwest Atlantic continental shelf from Cape Hatteras to Georges Bank (Fig. 1), is characterized by a productive benthic ecosystem [...]

Oyster population maintenance and growth require a sufficient larval supply competent for metamorphosis and settlement. Larval performance, in terms of growth, development, survival, and metamorphic success, determines the capacity for a larval cohort to effectively settle and establish into an existing population. Exogenous factors influencing larval development include temperature, salinity, food quantity, and food quality. A sufficient diet, composed of balanced protein, lipids, and carbohydrates to meet larval nutritional demands, is required to promote successful metamorphosis. To evaluate the influence of these exogenous factors on oyster settlement potential in Delaware Bay, a well-established biochemically based Crassostrea gigas (Thunberg, 1793) larval model was adapted to simulate Crassostrea virginica (Gmelin, 1791) larval performance under in situ environmental conditions measured during the 2009 to 2011 reproductive seasons at 10 sites across the salinity gradient of Delaware Bay. Variation in the initial egg size and lipid content, and larval food assimilation efficiency was incorporated into the model to represent potential within-cohort phenotypic variability. The middle portion of Delaware Bay along the New Jersey shoreline, bridging the 15-salinity line, generated the most successful larvae each year, whereas the low-salinity reach, on the Delaware side, and Nantuxent Point Reef had more variable success. Survivorship was a function of adequate temperatures and salinities, sufficient food quantity, and favorable food quality defined in part by the protein-to-(lipid-plus-carbohydrate) ratio. Most settlement was predicted by the model to occur between July and September of each year. To validate the model, estimated settlement windows were compared with calculated settlement windows derived from recruitment observations on yearly shell plants. Modeled and recruitment-derived settlement windows agreed well with each other and verified the capacity of the model to accurately forecast in situ larval performance. The oyster larval model, based on measures of lipid, protein, and carbohydrate, successfully passed an important field test, demonstrating the potential of such biochemically based models to reliably evaluate larval performance under real-world conditions. KEY WORDS: Crassostrea virginica, larvae, eastern oyster, performance model, settlement window, biochemical, food, INTRODUCTION Successful oyster population maintenance and growth require a consistent larval supply capable of settlement onto existing beds over time (Coen & Luckenbach 2000, Mann & Powell 2007, Lipcius et [...]

Oyster reefs are a primary source of calcium carbonate in most estuaries and the habitat complexity provided by these reefs supports a diversity of estuarine life. The available surface area of hard substrate on an oyster reef is critical for population maintenance, as this substrate provides habitat for the settlement of new oyster recruits. On natural reefs, the primary substrates available for settlement by spat include live oysters, boxes (dead articulated valves), and cultch (single oyster shells and shell fragments). Data from a long-term oyster monitoring program from February 2016 to October 2020 were used to generate a monthly time series of the percent of total carbonate weight contributed by live oysters, boxes, and cultch at four reefs in the Mississippi Sound. These percentages vary spatially and temporally. When reefs were healthy, live oysters dominated the total carbonate. During the study period, two mass mortality events occurred. Following the 2016 mortality event, the substrate contributing the majority of the carbonate shifted from live oysters to cultch at all reefs sampled. Three reefs exhibited a modest population recovery following the mortality event only to be impacted by a second, more severe mass mortality event in 2019. Boxes usually had the highest number of spat per gram carbonate at all reefs. The potential surface area (PSA) of each substrate type was calculated by comparing the available surface area of each substrate type and the number of spat that settled on each substrate. Correction factors were generated to account for larval settlement preferences and used to either debit or credit the PSA to calculate effective surface areas (ESA) for each substrate type. Cultch offered the highest ESA at Pass Christian, and cultch ESA was higher than box ESA at Henderson Point and Between Bridges reefs. At Bay St. Louis, box ESA was twice that of cultch. These results suggest cultch and boxes are more valuable settlement substrates than live oysters in this region under the conditions present during the survey time frame, which was characterized by low oyster abundance and aperiodic mass mortality. The unbalanced ESA, in which live oyster ESA is underrepresented, represents an unhealthy condition and demonstrates that a critical period exists for reef recovery after mass mortality in which the timely coincidence of increased box ESA and larval availability is essential for rapid recovery from mass mortality. KEY WORDS: Crassostrea, effective surface area, oyster reef, recruitment, mass mortality, population dynamics, recovery, INTRODUCTION The desperate state of the eastern oyster (Crassostrea virginica) over much of its US East- and Gulf-coast range, the accompanying loss of reefal area, and the responding investment into [...]

Organisms increase in size over time (age) due to excess assimilation over metabolic (respiration) energy demands. Most organisms reach a maximum size with increasing age as gain and loss balance. The von Bertalanffy length-at-age relationship, which is commonly used in fishery assessment calculations, imposes such a maximum size. However, some fished species, such as ocean quahogs, Arctica islandica, are long lived and continue to grow at old age. The Tanaka age-at-length relationship has continued growth at old age, but is rarely used in stock assessment models. A modified form of the von Bertalanffy model is presented, which mimics the continued growth at old age of the Tanaka model by allowing the growth parameter (K) to decline with age. This form is suitable for inclusion in stock assessment models based on von Bertalanffy. The proposed model matches Tanaka curves with precision appropriate for the scatter of data used to fit the curves. The observations of ocean quahog length at age and growth rate from New Jersey and Georges Bank demonstrate the ability of the modified von Bertalanffy relationship to represent continued growth at old age for this fished species. Simulated data generated with continued growth at old age were fit with the Stock Synthesis model (SS3). Results comparing traditional and modified growth relationships showed that the original von Bertalanffy model can reasonably approximate modest nonasymptotic growth as long as the number of observations is sufficient to constrain the parameter values. KEY WORDS: Arctica islandica, shellfish fisheries, length-at-age, growth model, ocean quahog, Bertalanffy growth, INTRODUCTION Most organisms increase in size over time in response to excess assimilated energy above metabolic demands, often termed scope for growth (Barilleet al. 1997, Munroeet al. 2013, Sanders et [...]

Warming of the Mid-Atlantic continental shelf has resulted in a range shift of the Atlantic surfclam, Spisula solidissima, north and offshore into waters still occupied by ocean quahogs (Arctica islandica). An ecotone, a boundary transitioning between neighboring ecological systems over a wide range of space and time, now exists over much of the offshore range of the surfclam in which surfclams and ocean quahogs co-occur. Regulations prohibit fishers from landing both species in the same catch, limiting fishing to locations where the target species can be sorted on deck. An at-sea survey sampling 50+ stations in the overlap region was conducted in September 2021 with the purpose of mapping fishable concentrations of surfclams and ocean quahogs. Size frequency and density data of both species were assessed along with environmental parameters. Species overlap between surfclams and ocean quahogs was most prominent in the 40- to 55-m depth range. Density of surfclams shifted within this depth from surfclam dominant in less than 40 m to ocean quahog dominant in greater than 60 m. Atlantic surfclam length increased with increasing summer bottom water temperature, whereas densities remained stable, indicative of proportionately larger but fewer animals in warmer inshore waters. Ocean quahog size metrics and densities, on the other hand, remain relatively unresponsive to temperature and invading Atlantic surfclam populations and instead increase in size with higher latitude. Large ocean quahogs, in particular, exhibit a distinct correlation with high latitude but fail to do so with other environmental variables. This analysis emphasizes the potential for economic disruption of fisheries as climate change pushes surfclams further into the range of the ocean quahog and highlights the need for regulatory changes to allow mixed catches and landings. The study also emphasizes the importance of the relative rates of transgression and regression of range boundaries by abutting faunas in determining the degree of influence of the ecotone between them on the benthic community structure of the continental shelf. KEY WORDS: Spisula solidissima, Arctica islandica, Atlantic surfclam, ocean quahog, range shift, ecotone, climate change, surfclam fishery, range boundary, species interaction, INTRODUCTION The Northwest Atlantic Ocean is a bellwether of climate change, as its warming trends far exceed those observed throughout oceans around the world (Pershing et al. 2015, Kavanaugh et [...]

The ocean quahog, Arclica islandica (Linnaeus 1767), is a commercially important species along the western Atlantic continental shelf. It is a long-lived species, frequently reaching ages over 200 y. Uniquely, it is one of the few bivalves to display sexual dimorphism, in that females grow to sizes larger than those of males. This phenomenon is believed to occur because males reach sexual maturity before females and, thus, have slower growth rates earlier. The growth rates of A. islandica from four sites across the Mid-Atlantic Bight. Georges Bank, Long Island (LI), and north and south of the Hudson Canyon, were measured to determine patterns in growth between males and females. Females begin to outgrow males between the ages of 5 and 15 y at sizes 50-55 mm, though this varies amongst the sites and between decades. Each of the sites is unique in some way. but three sex-dependent growth dynamics are observed. Most commonly, the two sexes diverge in size with females outpacing males in growth rates after the first 5-15 y of life. This outcome occurs at all sites and is generally the most common outcome across decades. In a few cases, female growth rates outpace the males very early in ontogeny. Such cases occur at two sites north of Hudson Canyon, both on the LI continental shelf. Most rarely, the two sexes maintain similar growth rates. This is observed for a few decades at the most southern site. In the population as a whole, these rare outcomes have limited influence on the population so that female-to-male ratio consistently increases with increasing size. This sexually dimorphic growth is not caused by protandry, nor is it compensation for a differential mortality rate between the sexes. Cases where males grow as fast as females may be just as easily indicative of a constraint on female growth as a facilitation on male growth. Egg sizes in A. islandica are larger than those of most other bivalves with planklotrophic larvae. Accordingly, another viable hypothesis is that differential growth is an adaptation to support the large egg sizes in females, where larger female size is essential to counterweigh the consequent reduced fecundity due to larger egg volume. KEY WORDS: ocean quahog, Arclica islandica, sexual dimorphism, Mid-Atlantic Bight, growth rate, INTRODUCTION The ocean quahog (Arctica islandica) is an infaunal, non-colonial species that ranges from Newfoundland, Canada, to Cape Hatteras, NC. USA, and from the Bay of Cadiz in Spain to [...]

The Atlantic surfclam (Spisula solidissima) is an important commercial fishery resource on the U.S. MidAtlantic continental shelf. Although the 2016 stock assessment found that surfclams are neither overfished nor is overfishing occurring, uncertainty in the scale of spawning stock biomass persists. As a consequence of this uncertainty, the MidAtlantic Fishery Management Council (MAFMC) lowered the acceptable biological catch in 2016. A simulation analysis was developed for Atlantic surfclam to estimate the overfishing risk associated with the catch recommendation and its adherence to the MAFMC risk policy. Operating models conditioned on the 2016 stock assessment model structure generated simulations of the surfclam population, with alternative models to represent uncertainty in steepness (h) of the stock-recruitment curve and natural mortality (M). Simulations were forecasted under a variety of management procedures and evaluated with estimation models that spanned uncertainty in h and M. Results showed that current management decisions are more conservative than the stated risk-tolerance policies, though overestimating steepness in assessment models could lead to the misrepresentation of an overfished stock as within management thresholds. Further analysis evaluated future economic viability of the fishery by estimating proportion of lishable clam patches given forecasted biomass and historical observations of clam density. The proportion of fishable patches able to support fishery economic sustainability was generally stable despite biological uncertainties, though declined with increasing fishing pressure. This work contributes to the efforts to evaluate environmentally and economically sustainable fishery management strategies. KEY WORDS: Atlantic surfclam, stock assessment, forecast, uncertainty, risk-tolerance, INTRODUCTION Many sources of uncertainty impact determination of fishery management targets [e.g., acceptable biological catch (ABC)] for all managed commercial and recreational fisheries (Rosenberg & Restrepo 1994. Roughgarden & Smith [...]

Populations of the eastern oyster (Crassostrea virginica) have experienced declines from overfishing and disease throughout much of its US range, though development of maximum sustainable yield (MSY) management criteria has been elusive. This is due in part to the discordance between oyster spawning stock and recruits, as the classic stock- recruitment model does not account for the requirement of shell substrate on which recruits settle. This issue was recently addressed with the development of a surface area-recruitment model, which is herein incorporated into a simulation analysis to estimate MSY-based reference points for C. virginica in the Delaware Bay. Simulations demonstrate that at low natural mortality, fishing mortality (F) may be sustainable at values between 10% and 15%; however, if disease or other mortality-enhancing processes occur, the margin of error in fishing is small and may quickly lead to population and reef collapse, emphasizing a precautionary F < 10%. The MSY-based reference points generated here provide rebuilding goals for the oyster fishery and reef management on fished and unfished reefs and the framework from which shell-planting can be incorporated and optimized in the future. Si les populations d'hurtres americaines (Crassostrea virginica) ont connu des declins dus a la surpeche et aux maladies dans la majeure partie de l'aire de repartition americaine de l'espece, l'elaboration de criteres de gestion bases sur le rendement equilibre maximal (REM) demeure problematique. Cela est du en partie a la discordance entre le stock reproducteur d'hurtres et les recrues, le modele stock-recrutement classique ne tenant pas compte de l'exigence d'un substrat de coquilles pour l'etablissement des recrues. L'elaboration recente d'un modele reliant la superficie au recrutement tente de resoudre ce probleme, et nous integrons ce modele a une analyse de simulation pour estimer des points de reference bases sur le REM pour C. virginica dans la baie du Delaware. Les simulations demontrent que, quand la mortalite naturelle est faible, la mortalite par peche (F) peut etre durable a des valeurs entre 10% et 15%, mais que si des maladies ou d'autres processus qui rehaussent la mortalite ont cours, la marge d'erreur associee a la peche est faible et pourrait rapidement mener a l'effondrement de la population et des recifs, d'ou l'importance, pour une approche prudente, de valeurs de F < 10%. Les points de reference bases sur le REM issus de la presente etude offrent des objectifs de reconstitution pour la peche a l'hurtre et la gestion des recifs exploites et non exploites, ainsi qu'un cadre a partir duquel le depot de coquilles peut etre incorpore et optimise. [Traduit par la Redaction], Introduction The use of biological reference points for sustainable management of commercial fisheries is well ensconced in the management of US federal fisheries, wherein achieving maximum sustainable yield (MSY) is [...]

A sustainable fisheries model for the estimation of harvests of oysters, maintenance of cultch, and restoration of reefs has been applied across the northern Gulf of Mexico. Oyster density and size, and cultch density are provided from surveys by State agencies. The model simulates oyster growth and mortality, and cultch loss. Reef shell mass is increased when oysters die in place and diminished when oysters are removed by fishing. Harvest is estimated as sacks of live oysters that can be removed while retaining reef quality (i.e., reef cultch mass, oyster density). Application of oyster density and cultch density standards (OCS) for fishing at maximum sustained yield shows little sustainable harvest on Public Grounds in the northern Gulf of Mexico. Model heuristics focus discovery of negative and positive feedbacks to reef demise or persistence. Oyster reef dynamics are viewed as persistent processes within negative and positive feedback loops, and occasional shifts between them. A negative feedback loop is set up on poor-quality reefs. Lack of cultch and a paucity of adult oysters diminish recruitment and shell replenishment. In the absence of recruitment, natural shell loss ultimately drives the reef to extinction. A positive feedback loop is set up on reefs with adequate cultch and an abundance of oysters. There, oysters set. survive critical early stages in boxes and reef interstices, reproduce, grow to a large size, and die in place. Harvests on high-quality reefs beyond that which maintains reef cultch and spawning stock, and consequential recruitment failure cause a shift from a positive to a negative feedback loop. Likewise, restricting harvests and planting cultch on poor-quality reefs, together with successful recruitment, promotes a shift from a negative to a positive state. Results of no-net loss of cultch (NNL) and OCS simulations suggest two distinct management approaches. The NNL promotes the sustainability of reef shell and assumes that a stable shell bed promotes a stable living resource, whereas OCS explicitly promotes the sustainability of cultch and stock at msy (maximum-sustainable-yield). Furthermore, OCS, which incorporates optimal reef cultch and oyster density standards, provides restoration benchmarks. Achieving sustainability at msy will simultaneously promote habitat restoration, restoration of ecosystem services, and a sustainable oyster fishery. KEY WORDS: Crassostrea virginica, maximum sustainable yield, reference points, cultch management, fishery management, eastern oyster, INTRODUCTION The eastern oyster Crassostrea virginica, is responsible for creating one of the primary habitat types in the estuaries of the northern Gulf of Mexico. The animal supports a commercial [...]

The implementation of a definitive measure of dredge efficiency for analyzing shellfish survey data substantially improves the estimation of abundance. Commonly, depletion experiments are used to estimate gear efficiency and [...]